GPS-Tracked Buoy for Water Level Measurements

With the advancement of differential, kinematic GPS techniques, water level measurements with the accuracy of centimeter- or decimeter-level is possible when buoys are placed close to a coastal base station or at distances of thousands of kilometers from shore, respectively. Applications of these techniques to observe tides and waves and to detect tsunamis have been demonstrated. This paper will first briefly review existing water level measurement methods, the needs for coastal and open ocean water level measurements, previous GPS buoy experiments, and GPS measurement uncertainties and precision positioning techniques. These will then be followed by a brief description of the application of GPS buoys in hydrographic surveying and the development of a real-time water level reporting GPS buoy to support US National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS) nautical charting mission. The present measurement system consists of a portable spherical buoy and a shore base station. Utilizing Real-Time Kinematic (RTK) differential GPS technique, accurate water level data were collected and reported in real-time at six- minute intervals in complying with NOS water level measurement standards. The buoy’s motion sensors provide automated corrections for wave-induced buoy motions. Several field tests conducted near NOAA water level stations have shown that the root-mean-square (rms) of differences between the two measurement systems is on the order of 2 cm. The buoy also provides surface wave information. The portability and the accuracy of the system offer possibilities for other applications in coastal waters. Future enhancements include satellite data telemetry and monitoring, option for post processing, and solar power supplementation.